Single-wheel landing gear with tail skid and small stabilizing skis on the wing tips. ALL landing gear elements are
completely retractable. At the time world new only semi-retractable gear with wheels going backwards and half-hide in the wing.

Water evaporative cooling system inside the wing surface, like in some racer aircraft.

Elevator controls had adjustable reduction number, allowing pilot to adjust force on the control stick depending on the
aircraft speed and flight conditions. This system was 'invented' again in
1944 by
TsAGI
and LII...

Stal-6 successfully passed all tests during Fall
1933. Pilots A.B.Yumashev and
P.M.Stefanovskij proved that 'one-wheel' landing gear is as good as conventional on takeoff, landing and taxi. Speed on ground level
was 420km/h, 150km/h faster than typical speed of contemporary fighters.

Interestingly, that VVS
authority already pushed aircraft builders to increase speed of their aircraft, but faced
'organised resistance'. Manufacturers did not believe that substantial speed increase is
possible in short time. They interpreted rumors about Stal-6 as a 'duck'.

On official meeting with K.E.Voroshilov and G.K.Ordzhonikidze representative
of the GUAP tried to prove that fighter with speed
400-450km/h is impossible. He was corrected that such aircraft (Stal-6) already exists
and its designer (R.L.Bartini) is present on the meeting.

GVF received an order to build fighter
based on Stal-6 design. Project was designated Stal-8...

There was a widely used argument against evaporative cooling systems - vulnerability.
But... this is a common feature of water-cooled engines (except special cases with
armor-shielded cooler like Il-2). Large area of
cooling surface is not a weakness - higher probability of combat damage is compensated by
relatively lower impact of a hole on the system.

During trials of
Stal-6 A.B.Yumashev performed few flights with
imitation of a cooler combat damage. Each flight lasted more than 30min! Taking into
account that flight endurance of (most) Soviet WW-II single-engine fighters was close to
1hour, damaged fighter could not only return home safely, but do it after the fight
is over.

There also was a room for improvement - Stal-6
had single-unit cooler, while segmented system could be even less sensitive to damage.